El modelo del Code civil français

When a particle is suspended in a medium, there are a number of forces acting on it which depend upon the nature of the particle as well as that of the medium. It is well known that the different states of matter are the result of varying strengths of the intermolecular attractions. In the case of solids, the intermolecular forces are quite strong, giving them a particular form and maintaining that shape. The intermolecular forces in a liquid are strong enough to restrain it to a particular volume without giving any shape and the liquids thus adopt the shape of the vessel in which they are contained.

20

The intermolecular forces present in a gas are the weakest of the three states of matter.

As a result, gases do not have any shape and occupy all the space available to them.

Solids and liquids have a surface which acts as a boundary layer between them and their surroundings, keeping them confined in a specific space. Gases, however, are devoid of any such surface. The surfaces of solids and liquids thus exist due to the presence of strong intermolecular forces. A more appropriate term for a surface is interface. Due to the presence of a distinctive phase boundary, the molecules of solids and liquids present at the interface behave differently than the molecules in the bulk. The molecules in the bulk are surrounded by similar molecules in all directions, so that the forces on any single molecule in the bulk cancel. On the other hand, the molecules present at the interface are subject to forces in two opposite directions, firstly attractive forces towards the interior of the solid or liquid and secondly the force from the outer phase. This second force can be attractive or repulsive depending upon the nature of the medium and the particle. As a result of the different kinds of forces acting on them, the surface molecules acquire a certain amount of energy which is proportional to the net magnitude of the forces acting on them. This energy is equivalent to the work needed to separate the surface molecules and is called interfacial energy. As the name suggests, this is a property of surface or interface molecules only [38]. If the attractive forces from the outer medium are stronger, the molecules at the interface would tend to move towards the outer medium. This would eventually result in the dissolution or miscibility of the two phases. In the case of disperse dye particles (which are essentially hydrophobic) in an aqueous medium, the outer medium exerts repulsive forces on the interfacial molecules, pushing them towards the centre of the particle. Thus disperse dyes cannot be dissolved in water, at least at low temperatures; instead, they form a colloidal dispersion [39, 40].

Aqueous solution Bulk dispersion

Figure 2.12 Schematic of the state of disperse dye in dispersion

21

A colloidal dispersion consists of dispersed particles in a continuous phase and is an intermediate state between solution and suspension. It is composed of two phases, the dispersed phase and the dispersion medium, corresponding to the solute and solvent in the case of a solution respectively. A phase is that part of a system which is chemically and physically homogeneous in itself. As disperse dyes do not have ionic groups and the particle size is greater than 1 nm, they do not form a solution in water. However, the size of disperse dye particles is less than 1000 nm which is smaller than the size of particles in a suspension and the presence of polar groups makes them slightly soluble in water so that the system is not classified as a suspension either. Thus, it can be said that the state of the disperse dye in water is a dispersion interspersed with some small regions of dye solution as shown in Figure 2.12.

According to the concept explained above, the attractive forces between the different molecules of disperse dyes predominate resulting in the formation of aggregates. Thus a disperse dye in water can be thought of as being surrounded by a hostile environment and other disperse dye molecules may be regarded as friendly; by attaching with these friendly entities, disperse dye molecules can create a safe haven for themselves where they are protected from the outside hostile environment. This happens when the molecules randomly collide with each under the influence of Brownian movement as the molecules should be close enough for the London dispersion forces to be effective.

A solution is obtained only when the solute and solvent have a similar nature.

However, in the case of a disperse dye, which is quite dissimilar to water, a solution is not possible because if the molecules of disperse dye leave the surface they face a hostile environment in the bulk where they have to do more work to remain suspended.

The lowest energy state is only possible when they remain closely attached to molecules of a similar nature thus giving rise to the aggregates of disperse dye.

To obtain a solution of the disperse dye some other arrangements have to be sought.

The first thing is to ensure that the size of the particles is small. There is a limit to the smallest particles size which can be obtained for disperse dyes. However, there has been significant progress since the first disperse dyes were synthesised, and the size of disperse dye particles has been reduced from 2 - 4 µm to currently less than 1 µm. This has still not reached the domain of the size of the particles in a solution. As the size of the particles decreases below 1 µm, the total surface area of the particle increases. This means that there is a larger number of molecules at the surface of a small particle than a large particle resulting in a higher interfacial energy and instability of the smaller

22

particles. For a macroscopic system, the interfacial energy of a phase and the number of molecules at the interface is negligible compared to its chemical potential and the number of molecules in the bulk. However, in colloidal systems, the number of interfacial molecules is not negligible when compared with the number of bulk molecules. This characteristic confers special properties on the colloids, one of which is colloidal solubility.

50 100 150 200

Specific surface (cm-1 )

1.0 2.0 3.0

Particle diameter (m)

Figure 2.13 Relation between surface area and size of a spherical particle

Solubility is the capacity of a medium to contain another compound such that the result is a homogeneous medium. The term solubility indicates that the solute and solvent are in a state of equilibrium under a particular set of conditions. Colloidal solubility and macroscopic solubilities are different. Macroscopic solubility can be used in those cases where the solubility is independent of the particle size. Below a diameter of 1 µm, the specific surface increases very rapidly as shown in Figure 2.13. These small particles have an increased intrinsic energy which makes them unstable. When dispersed in a solution, such particles tend to move towards a lower energy state which is towards the solution phase or towards coarser particles to reach a state of equilibrium.

A colloidal system is polydisperse in nature and is thus unstable. As the smaller particles disappear resulting in the formation of coarser particles, the solubility equilibrium also changes. The new equilibrium will be based on the coarser particles with lower solubility. Thus true solubility equilibrium cannot be reached in colloidal

23

systems and a solubility determined under a particular set of conditions will decrease with time. A disperse dye dispersion is inherently unstable and if it reaches equilibrium it ceases to be a dispersion. The solubility values of disperse dyes which are quoted in the literature are for macroscopic solubility only and should be considered with care before making any generalisations. Thus for a colloidal system the dissolution properties are more important than the solubility. Both solubility and rate of dissolution are higher in colloidal systems. Other important factors which influence the solubility of colloidal systems are:

 Particle diameter - small particles have higher solubility than large particles of the same compound.

 Ostwald ripening - this is the increase in size of the larger crystals of a compound, especially a sparingly soluble compound, as the smaller and more readily soluble crystals disappear [41]. The driving force for the Ostwald ripening is the difference in the solubilities of the particles in a polydisperse system. This creates a concentration gradient between smaller and larger size crystals and the compound transfers through the solution from higher energy to lower energy particles. This ageing process depends upon the size distribution, rate of dissolving, rate of crystal growth and the transfer through the solution.

These in turn are influenced by time and temperature. However, it should be realised that the particles of the disperse dye are not spherical and thus Ostwald formula is not strictly valid.

 Change in crystal modification - if the crystal shape differs much from the equilibrium shape, the rate of crystal growth increases. This happens when the disperse dyes are ground to provide a fine particle size and as a result the crystal structure is deformed. As the particles are mechanically ground, the internal stresses increase making the smaller particles highly unstable resulting in an increase in solubility as well as the Ostwald effect. The energy content of the particles is influenced by both the size and state of surface molecules.

 Activation of particles by mechanical stress during grinding - on grinding, the crystal particles undergo deformations resulting in thermodynamic and structural instability and in such a state the reverse process, that is the recrystallisation, becomes spontaneous. As the energy content of the particles increases, their solubility in water and fibre increases also.

 Change of crystal habit in the presence of surface active agents [42].

24

The above discussion makes it clear why the disperse dye dispersion is prone to destabilisation. While all of the above mentioned factors are related to causes that are beyond the control of a dyer and can only be managed during the synthesis of the commercial dye preparation, there are other factors which are under the control of the dyer. These include dye concentration, temperature, time, pH, electrolytes, auxiliaries, materials present on the fibre and stirring. Dye particle agglomeration increases with an increase in dye concentration, temperature and dyeing time. Sudden temperature changes can also induce crystallisation. However, dispersion stability does not depend upon pH within the range 4 -6. Metallic ions such as calcium, magnesium and copper are undesirable. Vigorous stirring can also have an adverse effect on the dispersion stability [43].